The invention relates to methods for controlling paper quality in the production of a web of paper.
In prior art paper and board machines, the stock is fed from the headbox to the former, usually to a joint run between two wire loops, where water is removed from the web as symmetrically as possible through both wires. The objective is a web which is symmetrical in relation to the longitudinal center level of the web. From the former, i.e. from the wire section, the paper or board web is led to the press section, where more water is removed from the web, and then to the dryer section, after which the web is reeled on the reel and, if necessary, taken to finishing.
The production of paper and board webs conventionally begins in the headbox by preparing a stock, into which fiber material and fillers as well as fines and additives are mixed. The stock system mixes the fibers and fillers as well as the potential fines and additives into a stock which is as homogeneous as possible, the stock to be fed into the headbox of the paper or board machine. In multilayer webs, several separate stock systems are also used for feeding different fiber suspensions either into a single headbox or several headboxes. The headbox spreads the stock suspension evenly onto the wire section, where dewatering and web formation begin. There are several different types of prior art wire sections or formers: Fourdrinier formers, hybrid formers and gap formers. Board machines may have several wire units. In prior art solutions, it is possible to control the fiber and filler distribution in the thickness direction of the web in a limited manner only, for instance through the placement of dewatering elements on the former and through the use of vacuum. Fillers often accumulate on the web surfaces during the dewatering phases.
The control of filler distribution in the thickness direction of the paper web is a desirable feature in many senses. However, prior art solutions do not provide a simple and inexpensive solution for this. The need to control the filler distribution may be caused by factors such as a need to control the symmetry (absorption, roughness) between the surfaces of the paper web or by a need to control the surface properties of the base paper in coating carried out in conjunction with finishing, for instance. However, the conventional problem has been to identify the magnitudes and directions of variables influencing distribution control as well as the mutual interaction between various impacts. This control is further complicated by the fact that the simultaneous optimization of the various sub-variables of paper web quality does not succeed as desired, which means that compromises need to be used.
One of the problems of prior art solutions in on-line fine paper applications is that the on-line process does not give information on the filler distribution of the web, but information on the successfulness of the filler distribution is obtained after a delay of approximately one day, because the filler distribution can only be determined on the basis of analyses made of a complete web.
Prior art arrangements include the measurement of water quantities on the wire section, but information obtained in this way has not been utilized directly but it has primarily been used for gathering information and not as a basis for readjustments.
With regard to prior art solutions, reference is made to U.S. Pat. No. 5,825,653, which presents a control method for the wire section based on a flow model, where the wire section is controlled by means of flow calculation. In this known arrangement, a physical flow model is established, based on wire dewatering and on the flow state of the stock suspension so that wire dewatering is measured at several locations of the wire section by measuring the quantity of water removed at the different locations, and the flow state of the stock suspension is defined by means of the stock jet velocity, wire speed and stock consistency. Paper quality is monitored at the dry end of the paper machine. The model defines a target flow state and the difference between the current flow state and the target flow state, which gives a cost function which is used for defining new control and set values so that the target flow state could be reached. This known solution hence requires the creation of a physical flow model, which makes the method rather complicated and requires considerable know-how.
Prior art reference is also made to U.S. Pat. No.5,879,513, which presents a method and equipment for dewatering on the wire and/or press section of a paper machine or similar machine. In this known method, at least one suction pipe is equipped with at least one slot, and the suction pipe is connected to a vacuum source in order to create a vacuum in at least this suction pipe, and the felt, wire or similar fabric is transferred in this method over the slot in the suction pipe so that the vacuum removes water from the felt, wire or similar fabric. In this known method, at least the quantity of water removed in the suction pipe is measured, and the vacuum capacity is controlled on the basis of the measured information by means of control equipment.